Ms. Bianchi, of course you have every right to protest this memorial and to erect any signage you choose. In this case, however, I fear that you are tilting at windmills. The men erecting this structure have all sworn a sacred oath to honor and protect the memories of their confederate ancestors. They will not relent nor will they yield.
I think your efforts would prove more fruitful if you solicited the aid of pandering politicians such as Mitch Landrieu of New Orleans or city councils comprised of the perpetually offended in cities where changing demographics have placed them in power.
This memorial stands on private land and unless you also want to remove constitutional protections; you will lose this fight.
Also, I question your stated level of support in this endeavor. The results of a recent poll suggest approximately seventy percent of the locals support this structure.